This invention is based on Japanese application 2000-165864 filed Jun. 2, 2000, which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a light emitting device, and more particularly to a light emitting device suitable for use, for example, as white light sources for backlight or frontlight in liquid crystal display panels.
2. Description of Related Art
Various light emitting devices for backlight in full color liquid crystal display panels are known. Generally, a light emitting device (LED) includes an LED chip array comprising an LED for red (R), an LED for green (G), and an LED for blue (B). A light guide section is generally provided such that light from the LED chip array enters therein through an incident face. The light is guided so as to propagate through the inside of the light guide section, thereby permitting planar backlight to be emitted through an outgoing face from the light guide section. Each of the LEDs for respective colors R, G, and B has a narrow-band emission spectrum. Therefore, upon light emission from each LED for R, G, and B, lights of R, G, and B from the respective LEDs can be mixed together to supply white backlight to a liquid crystal display panel. One such example is shown in Japanese Patent Laid-Open No. 329044/1999.
Radiation characteristics of each light emitting device varies from position to position and the speed of deterioration varies for each light emitting element. This tends to change the color balance over time. Light emitting elements having poor radiation characteristics can generally cause lowered power output and have shortened service life, which is undesirable.
Some light emitting elements, such as G and B, have both positive and negative electrodes on light emitting faces thereof, which can increase the number of bonding elements coupled to the light emitting elements. With more bonding elements coupled to the light emitting elements, a compact packaging arrangement is difficult to achieve.
Accordingly, it is an aspect of embodiments of the invention to provide a light emitting device in which homogeneous radiation characteristics can be obtained and, thus, no significant change balance in color results from the elapse of time.
It is another aspect of embodiments of the invention to provide a light emitting device in which improved radiation efficiency is obtained and unfavorable phenomena such as lowering in power output and shortening of service life of light emitting elements can be avoided.
It is a further aspect of embodiments of the invention to provide a light emitting device having a compact structure.
According to a first feature of the invention, a light emitting device includes an insulating base having positive and negative leads, respectively, provided on top and bottom surfaces thereof. A LED chip array is arranged on the negative lead on the top surface of the insulating base. The LED chip array is electrically connected to the positive and negative leads on the top surface of the insulating base. A first metal connection connects the positive leads on the top and bottom surfaces of the insulating base; and a second metal connection connects the negative leads on the top and bottom surfaces of the insulating base.
According to a second feature of the invention, a light emitting device includes a pair of metal layers provided respectively on the upper and lower surfaces of an insulating base. A plurality of light emitting elements are arranged on the metal layer provided on the upper surface of the insulating base, and a metal connection connects the pair of metal layers to each other at a position where at least one of the of the plurality of light emitting elements is disposed.
According to this construction, heat generated from the plurality of light emitting elements is released through the metal layer provided on the upper surface of the insulating base into the air, and, in addition, is transferred through the metal connection to the metal layer provided on the lower surface of the insulating base and is then released into the air. Further, since the radiating surface is increased, the radiation efficiency can be improved.
According to a third feature of the invention, a light emitting device includes a substrate comprising a plurality of leads provided on an insulating base. A plurality of light emitting elements are arranged on a base line along the surface of the substrate in its predetermined direction. A plurality of bonding wires connect the plurality of light emitting elements to the plurality of leads in the predetermined direction or on one side relative to the base line.
According to this construction, in the connection of the plurality of light emitting elements to the plurality of leads through bonding wires, this connection is carried out in such a state that the bonding wire has been rendered eccentric. This can reduce the size of the device in a direction perpendicular to a direction in which the plurality of light emitting elements are arrayed. The plurality of light emitting elements comprise a first light emitting element having first and second electrodes on its light emitting face side and a second light emitting element having a first electrode on its light emitting face side and a second electrode on its side remote from the light emitting face. The term xe2x80x9clight emitting elementxe2x80x9d as used herein means a bared chip, such as LED (light emitting diode) having first and second electrodes. The term xe2x80x9csubstratexe2x80x9d refers to, for example, a printed board formed by providing leads connected to first and second electrodes on a base by a circuit printing method, and a substrate having a lead frame structure formed by placing a lead frame, corresponding to leads connected to first and second electrodes, within a mold and pouring an insulating material into the mold.
According to a fourth feature of the invention, a light emitting device includes a pair of metal layers provided respectively on the upper surface and lower surface of an insulating base. A plurality of light emitting elements are arranged on a base line along the surface of the metal layer, in its predetermined direction, provided on the upper surface of the insulating base. A metal connection connects the pair of metal layers to each other at a position where a predetermined light emitting element out of the plurality of light emitting elements is disposed. A plurality of bonding wires connect the plurality of light emitting elements to the metal layer, provided on the upper surface of the insulating base, in the predetermined direction or on one side relative to the base line.
According to this construction, heat generated from the plurality of light emitting elements is released through the pair of metal layers provided respectively on the upper and lower surfaces of the insulating base into the air. In addition, in the connection of the plurality of light emitting elements to the plurality of leads through bonding wires, this connection is carried out in such a state that the bonding wire has been rendered eccentric. This can reduce the size of the device in a direction perpendicular to a direction in which the plurality of light emitting elements are arrayed.
According to a fifth feature of the invention, a light emitting device for driving a plurality of LED chips disposed in an array to emit a mixed light composed of lights emitted from the plurality of LED chips, an insulating base having an upper surface and a lower surface. An LED chip connection lead is provided on the upper surface of the insulating base. A power supply connection lead is provided on the lower surface of the insulating base. A link lead connects the LED chip connection lead to the power supply connection lead between the upper and lower surfaces of the insulating base. The LED chip connection lead comprises a plurality of separate leads connected respectively to the plurality of LED chips and a common lead, to which the plurality of LED chips are connected by common connection and which is loaded with the plurality of LED chips, for absorbing heat generated from the plurality of LED chips.
According to this construction, heat generated from the plurality of LED chips is released, through a common lead in the LED chip connection lead provided on the upper surface of the insulating base, into the air, and, in addition, is transferred through the link lead, to the power supply connection lead provided on the lower surface of the insulating base, and is then released into the air. Further, since the radiating surface is increased, the radiation efficiency can be improved.
These and other aspects and features of this invention will be described in or apparent from the following detailed description when taken in conjunction with the accompanying drawings, which are a part of this disclosure and which illustrate, by way of example, preferred embodiments of the invention.